This invention relates to mirrors for use in laser resonators and, more particularly, to mirrors operable in the extreme ultraviolet for use with a free electron laser (FEL). This invention is the result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
There are numerous applications which require tunable sources of coherent radiation operating in the extreme ultraviolet (XUV) wavelength range of 10-100 nm. FEL oscillators and amplifiers have produced radiation at wavelengths less than 500 nm. RF linacs, such as available at Los Alamos National Laboratory, can extend FEL operation below 100 nm. A high brightness electron beam developed at the Los Alamos National Laboratory can provide an electron beam to the linac that will more than meet requirements for FELs operating at XUV wavelengths as short as 10 nm. Further, FEL gain can be increased by providing long undulators to increase the number of undulator periods. The FEL gain can be further increased with an undulator geometry that provides equal, 2-plane magnetic focusing from a distributed quadrupole field or by sextuple focusing. Further, the magnitude of individual, random magnet errors that can be tolerated can be increased from below 0.1% to the order of 0.7% by periodic undulator segmentation and correction. However, a major problem with extension of the FEL to the XUV is the inherently low reflectance of available resonator mirrors for the FEL oscillator. The small-signal gain for the FEL decreases monotonically with decreasing wavelength to the 1/2 power. To balance the requirements on the electron beam, magnetic undulator, and resonator mirrors, a minimum acceptable resonator reflectance of 40% is adopted as a threshold resonator reflectance for XUV wavelengths. Even with this threshold value, the single-pass small-signal gain must exceed 625% just to reach an oscillation threshold. Similarly, with a mirror reflectance of 60%, a minimum small-signal gain of 280% is required.
Below 100 nm, normal incidence-type reflectors are not available with the required reflectance. Chemically vapor-deposited, single-crystal, silicon carbide mirrors have been produced with a reflectance of 40-50% for wavelengths between 60 and 220 nm, with the reflectance dropping to less than 10% for .lambda.&lt;60 nm. On simple metallic films, the 40% reflectance is obtained only for .lambda.&gt;250 nm, except for freshly deposited aluminum in ultra-high vacuum (.about.10.sup.-10 torr), which can produce a normal-incidence reflectance greater than 40% for wavelengths as short as 80 nm. For .lambda.&gt;120 nm, MgF.sub.2 overcoated Al films are capable of reflectances of 80% for normal-incidence radiation. Multilayer thin-film structures have yielded reflectances up to 60% only for XUV wavelengths near 15 nm and only over a narrow bandwidth limited to about 7%. Accordingly, normal-incidence mirror technology does not appear suitable for FEL extension to the XUV.
A single, cylindrical mirror configuration for use with soft X-rays, i.e. radiation with 5 nm.ltoreq..lambda..ltoreq.15 nm, has been suggested by A. V. Vinogradov et. al., "On Wide-Band Mirrors for Soft X-ray Range," 47 Opt. Commun, No. 6, pp. 361-363 (October 1983). The paper predicts that a coating of Rh will provide a reflectance greater than 40% at a wavelength of about 12 nm on a cylindrical surface, rotating the incident light through an angle of 180.degree. using total external reflectance. However, cylindrical surfaces introduce substantial astigmatism that degrades beam quality.
These and other problems of the prior art are addressed by the present invention and a FEL retroreflector is provided for use with XUV wavelengths in the range 10-100 nm.
Accordingly, it is an object to the present invention to reflect incident light of wavelength 10-100 nm through 180.degree. with a reflectivity of at least 40%.
It is another object of the present invention to reflect high-power laser beams with high reflectance over a broad bandwidth.
It is another object of the present invention to provide a retroreflector which does not introduce beam degradation such as astigmatism.
One other object of the present invention is to provide a retroreflector which can produce a collimated output beam.
Yet another object of the present invention is to provide a retroreflector with mirror surfaces effective to spread the incident beam for reduced energy absorption density.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.